The present invention pertains to the removal of soluble metal ions from solution and, more specifically, to apparatus through which an effluent solution containing soluble metal ions is continuously passed for treatment and removal of the metal ions.
In various types of industrial processes and plants, substantial quantities of soluble metallic ions are discharged in waste solutions, generally referred to as effluent. The effluent must be treated so as to either recover or at least remove these metallic ions from solution, prior to discharge of the effluent into the environment. For example, in the metal plating industry, rinse solutions and exhausted plating solutions contain residual amounts of soluble metallic ions. If economically feasible, it may be desirable to recover and recycle the metal remaining in solution. Otherwise, the residual metal must at least be removed from the effluent before it can be disposed of in an environmentally acceptable manner. The invention provides apparatus and a method for removal of such metal ions from even very dilute solutions thereof.
Presently, such soluble metallic ions are most commonly removed from effluent solutions by precipitation and settling, which involves separate treatment that is done on a batch basis. The disadvantages of this approach include incomplete and inefficient removal of the metal, as well as the commercial economics both in terms of time and expense which are involved. In accordance with the invention, it has been discovered that soluble metallic ions can be removed galvanically in an efficient, inexpensive and reliable manner by utilizing the novel apparatus and method of the invention.
Attempts have been made in the past to remove soluble metal ions by electrochemical processes. U.S. Pat. No. 3,859,195 provides apparatus comprising stainless steel and carbon electrodes provided in a "jelly-roll" configuration wherein the anode and cathode are insulated against metal-to-metal electrode contact. This apparatus, however, is connected to a power supply and a potential is established between the electrodes, whereby the metal ion is caused to plate onto the cathode. In contrast, the apparatus of the invention comprises electrodes in direct physical contact which utilize and/or require no application of exterior electromotive force or potential to effect and maintain plating of the soluble metal ions onto the acceptor electrode. The method of the invention utilizes a galvanic, rather than electroplating, process.
Although attempts have been made to remove soluble metal ions using a galvanic process, the invention provides novel improvements thereover. In previous attempts, aluminum electrodes were suspended in effluent solution contained in a tank, in order to effect galvanic plating of copper out of solution over a given period of time. Difficulties and disadvantages, however, are inherent in this batch type of prior art apparatus and method. As copper plates onto the aluminum electrodes, it accumulates to a degree which requires a replacement or removal. As the copper accumulates on the aluminum electrodes, the aluminum passivates, the process slows down and eventually will stop.
Furthermore, the time required to remove an acceptable amount of copper ions is too great to allow the method and apparatus to be effectively utilized in many commercial applications. This is believed to result because aluminum ions are trying to dissolve from the same electrode towards which copper ions are trying to diffuse in order to be plated out galvanically.
It has been advantageously and unexpectedly discovered that the apparatus and method of the invention achieve a greatly improved rate of removal of soluble metal ions from effluent solution in a manner which is continuous and requires a minimal amount of maintenance. In the present invention, aluminum ions leave the donor electrode while copper ions plate out on the acceptor electrode which is in physical contact with a donor electrode. Thus, the ions do not collide in the process of transfer and the apparatus and process of the invention are more efficient and much faster than the prior art.